Presently most bicycle transmission utilize front and rear sprockets having a series of sprocket drives of various diameters. These devices utilize derailleurs to move a drive chain between sprocket drives of selected diameters so as to provide the desired drive ratio. As is well known these types of transmissions, commonly referred to as derailleur systems, have many drawbacks. These systems are difficult to operate, they are noisy, they are delicate, and because of chain misalignment in some gears, they are inefficient.
For these reasons several attempts have been made to provide improved transmission, some of which involve the use of variable sprockets. Examples of these may be found in U.S. Pat. Nos. 3,850,044; 3,867,851; 3,994,180; 4,167,124; 4,493,678; 4,498,351; and 4,521,207. Other related expansible pulleys may be found in U.S. Pat. Nos. 885,710; 1,119,746; 1,144,381; 1,205,020; 3,279,271; and 4,068,539. For the most part these devices have failed to be commercially successful because of their complexity, high cost, and weight, and even more importantly, their difficulty of operation. It is therefore the object of the present invention to overcome these and other shortcomings and to provide a durable variable-speed drive unit, the operation of which is extremely simple and smooth.